Stars

Wow!

Stars evolve, or change, over time. It may take millions of years or
it may take billions of years for a star to complete its life cycle.

A star is a brilliantly glowing sphere of hot gas whose energy is produced by
an internal
nuclear
fusion process. Stars are contained in galaxies. A galaxy contains not
only stars, but clouds of gas and dust. These clouds are called nebulae, and
it is in a nebula where stars are born. In the nebula is hydrogen gas which is
pulled together by gravity and starts to spin faster. Over millions of years,
more
hydrogen gas is pulled into the spinning cloud. The collisions which occur
between the hydrogen atoms starts to heat the gas in the cloud. Once the
temperature reaches 15,000,000 degrees Celsius, nuclear fusion takes place in
the center, or core, of the cloud. The tremendous heat given off by the
nuclear fusion process causes the gas to glow creating a protostar. This is
the first step in the evolution of a star.
The glowing protostar continues to accumulate
mass.
The amount of mass it can accumulate is determined by
the amount of
matter available in the nebula. Once its mass is stabilized,
the star is known as a main sequence star.
The new star will continue to glow for millions or even billions
of years. As it glows, hydrogen is converted into helium in the core by
nuclear fusion. The core starts to become unstable and it starts to contract.
The outer shell of the star, which is still mostly hydrogen, starts to expand.
As it expands, it cools and starts to glow red. The star has now reached the
red giant phase. It is red because it is cooler than the protostar phase and
it is a giant because the outer shell has expanded outward. All stars evolve
the same way up to the red giant phase. The amount of mass a star has
determines which of the following life cycle paths the star will take.

MEDIUM STARS

As a red giant, the hydrogen gas in the outer shell continues to burn as the
temperature in the core continues to rise. At 200,000,000 degrees Celsius, the
helium atoms fuse to form carbon atoms in the core. The last of the hydrogen
gas in the outer shell is blown away to form a ring around the core. This ring
is called a planetary nebula. When the last of the helium atoms in the core
are fused into carbon atoms, the medium size star begins to die. Gravity
causes the last of the star's matter to collapse inward and compact. This is
the white dwarf stage which is extremely dense. White dwarfs shine with a
white hot light but once all of their energy is gone, they die. The star has now reached the black dwarf phase.

MASSIVE STARS

Once massive stars reach the red giant phase, the core temperature continues
to increase as carbon atoms are formed from the fusion of helium atoms.
Gravity continues to pull together the carbon atoms in the core until the
temperature reaches 600,000,000 degrees Celsius. At this temperature,
carbon atoms form heavy elements such as oxygen and nitrogen. The fusion and
production of heavy elements continues until iron starts to form. At this
point, fusion stops and the iron atoms start to absorb energy. This energy is
eventually released in a powerful explosion called a supernova. A supernova
can light the sky up for weeks. The temperature in a supernova can reach
1,000,000,000 degrees Celsius. This high temperature can lead to the
production of new elements which may appear in the new nebula that results
after the supernova explosion. The core of a massive star that is 1.5 to 4
times as massive as our Sun ends up as a neutron star after the supernova.
Neutron stars spin rapidly giving off
radio
waves. If the radio waves appear to be emitted in pulses (due to the
star's spin), these neutron stars are called pulsars. The core of a massive
star that has 10 or more times the mass of our Sun remains massive after the
supernova. No nuclear fusion is taking place to support the core,
so it is swallowed by its own gravity. It has now become a
black
hole which readily swallows any matter and energy that comes too near it.
Some black holes have companion stars whose gases they pull off. As the gases
are pulled down into the black hole, they heat up and give off energy in the
form of
X-rays. Black holes are detected by the X-rays which are given off as
matter falls down into the hole.